This invention relates to tube fittings for systems or equipment using tubing to conduct gasses or liquids and more particularly relates to tube fittings for retaining tubes used with vibrating equipment such as air compressors and hydraulic and pneumatic systems for transportation vehicles.
Reciprocating equipment such as air compressors operate at elevated temperatures, deliver a pulsating stream of fluid and vibrate considerably. Hydraulic and pneumatic systems used for controls and for brakes in trucks, buses, off-highway vehicles, locomotives and other transportation equipment also operate in hot environments. Components of these systems are subject to vibration caused by engines propelling the equipment and as a result of uneven roadways over which the equipment travels. Air compressors and other pneumatic and hydraulic equipment use fittings to connect tubing to conduct gasses and liquids to and from that equipment. Due to strong market competition, manufacturers are in a constant race to design and manufacture components, such as tube fittings for this type of equipment, more inexpensively, while maintaining a required level of quality and durability.
The various types of fittings currently in use all have their advantages, as well as offsetting disadvantages.
Compression fittings compress metal ferrules about a tube within the fitting to hold the tube. While they are one of the least expensive types of fittings, installation of tubes into compression fittings is difficult. FIG. 1 shows a compression fitting 20 having a body 22 with a bore 24 at one end surrounded by male threads 26. A nut 28 fits on the threads 26 of the body 22. A ferrule 30 is placed over a tube 32 which is to be held by the fitting 20. The ferrule 30 has sloped end surfaces 33 which allow it to be compressed by an inside surface 34 of the bore 24. The tube 32, with the ferrule over it, is inserted into the fitting body and the nut 28 is threaded onto the threads 26 of the body 22. The nut 28 is tightened with a wrench to compress the ferrule 30 around the tube, securing and sealing the tube.
When metal ferrules are used to crimp metal tubing, early tube failures often occur due to fatigue caused by machine vibration. Crimping also reduces the inside diameter of the tube, reducing flow capacity. Some compression type fittings use soft Teflon or silicone ferrules in place of metal ferrules. While the soft ferrules may minimize tube breakage due to fatigue, the soft ferrules are known to fail prematurely by wearing out and leaking. The soft ferrules also make assembly more difficult.
Flare fittings have been used to retain tubes for applications having significant vibrations, such as air compressors. A flare fitting 36 is shown in FIG. 2. The flare fitting 36 has a body 38 having a bore 40 at one end, surrounded by internal threads 42 forming a nut. The flare fitting 36 also includes a retainer 44 having external threads 46 which intermesh with the threads 42 on the body 38. The retainer 44 has a central hole within it into which tubing 48 is inserted. A flare 50 is formed in the end of the tubing 48 by a flaring tool to allow the nut at the end of the body 38, surrounding the tubing adjacent the flare, and an internal end 52 of the retainer 44 to clamp the flare 50 between them and hold the tubing 48 tightly within the fitting body 38.
Flare type fittings do not reduce tube life to the same extent as compression fittings, since they do not crimp the tubing and they also do not reduce the inside diameter of the tube. However, flare fittings experience failures due to fatigue at the areas of the flared tubing which contacts the members of the fitting. Additionally flare fittings are more expensive to manufacture and assemble because they tend to be larger in size and require the user to flare the tubing and are difficult to assemble.
Push-in tube fittings of the type shown in FIG. 3 allow a tube to be connected to the fitting by pushing the tube into the fitting. A push-in fitting 53 includes a body 54 and a gripper formed by axially extending fingers 56 of a cylindrical collet 58. Each of the distal ends 60 of the fingers 56 comprises a cam section. The cam section causes the fingers 56 to be deflected as the collet 58 is inserted into the body 54. The cam section 60 can move outwardly into an enlarged central cylinder formed by camming surface 62 within the body 54. An O-ring 64 is inserted within the body 54 at the end of the central cylinder 66 so as to form a seal about tubing 68 when it is inserted within the fitting 53.
When the tubing 68 is inserted through the collet 58, the fingers 56 are moved axially in an outward direction. The cam 60 bears against the camming surface 62 which allows the tube to be pushed within the body 54. However, the camming surface 62 causes the fingers 56 to compress about the tubing 68 to prevent it from being removed from the fitting 53 whenever a force attempts to pull the tubing 68 out of the collet 58. The force of the fingers 56 about a metal tubing causes the tubing or the fingers 56 to fatigue in areas where it is gripped throughout the use of the push-in fitting on equipment which is subject to vibration. This fatigue ultimately causes a failure of the tubing or the fingers 56.
Other push-in fittings include a fitting assembly having a retaining ring with gripping teeth about its periphery rigidly installed within the body to hold tubing. These fittings also include an O-ring within the fitting to act as a seal. One such push fitting 70, shown in FIG. 4, comprises a fitting body 72 and a nut 74 having internal screw threads which screw onto screw threads at the tube receiving end of the body 72. An O-ring seal 76 is mounted within a cavity 78 in an end of the body 72. A retaining ring 80 having gripping teeth 82 is mounted within a cavity inside the nut 74. A spacer 84 is inserted within the cavity of the nut 74 between the retaining ring 80 and the body 72 of the fitting 70. The spacer 84 protects the O-ring seal 76 from the gripping teeth 82. The surface of the spacer 82 adjacent the gripping teeth 82 is canted to allow room for the gripping teeth to bend as a tube 86 is installed.
The tube 86 is installed into the push fitting 70 by sliding the tube 86 through the nut 74 and the gripping teeth 82 of the retaining ring 80. The tube is held in place by the gripping teeth 82. An end 88 of the nut 74 holds the retaining ring 80 in place after the nut 74 is tightened onto screw threads 90 of the body 72.
Currently available push fittings which use retaining rings do not work well with metal tubing because the tubing fails due to fatigue where it is held by gripping teeth, such as gripping teeth 82 shown in FIG. 4. The gripping teeth of retaining rings normally do not fail because the retaining rings are made out of spring steel which is harder than the material of the tubing. Thus, available push fittings using retaining rings require the use of plastic tubing with them. Since vibrating equipment, such as air compressors and transportation systems, operate at elevated temperatures, the tubing must be made out of more expensive, high temperature plastics such as Teflon.
Among the many patents which disclose prior art fittings of the type described above are U.S. Pat. No. 3,999,783 Andre Legris issued Dec. 28, 1996; U.S. Pat. No. 4,867,489 Hiralal V. Patel issued Sep. 19, 1989; U.S. Pat. No. 5,024,468 Donald G. Burge issued Jun. 18, 1991; U.S. Pat. No. 5,230,539 Darwin C. Olson issued Jul. 27, 1993; U.S. Pat. No. 5,584,514 Michael A. Sweeney et al. issued Dec. 17, 1996; and U.S. Pat. No. 5,683,120 David J. Brock et al. issued Nov. 4, 1997.
In accordance with this invention, a fitting for retaining a cylindrical tube includes a fitting body having a hole extending through it. The body also has a cylindrical main cavity located at one end into which an end of the tubing will be inserted. A circular retaining shoulder is locatable on the tubing and within the main cavity when the tubing is placed in the fitting. A closure is installable at the open end of the main cavity to hold the shoulder within the main cavity. The closure has a circular hole within it to receive the tubing. An elastomeric seal is mounted within the main cavity between the shoulder and the closure. The seal has an inner diameter and an outer diameter which cause the seal to be compressed between the tubing and the wall of the main cavity.
The outer diameter of the retaining shoulder is sufficiently smaller than the diameter of the cylindrical main cavity so as to form a preselected shoulder clearance gap. The diameter of the closure hole is sufficiently larger than the outside diameter of the tubing so as to form a preselected closure clearance gap. Additionally, the axial length of the main cavity is sufficiently larger than the combined widths of the shoulder and of the seal to form a preselected axial gap. The shoulder clearance gap and the closure clearance gap allow sufficient side play between the tubing and the fitting and the axial gap allows sufficient axial play between the tubing and the fitting so that at least a portion of the shoulder can move freely between an end wall of the main cavity and the seal to an extent which minimizes fatigue on the tubing within the main cavity.
The retaining shoulder can be any of a number of possible structures. One preferred structure of the retaining shoulder includes a retaining ring having a plurality of inwardly extending teeth which allow a tube to be inserted into the fitting and engage and hold the tubing against withdrawal from the fitting. This structure also includes a washer located between the retaining ring and the seal. The outer diameter of the washer is the diameter of the shoulder in that the washer normally has an outer diameter larger than that of the retaining ring to protect the retaining ring from contact with the axial wall of the main cavity. The inside diameter of the washer is larger than the outside diameter of the tubing. As a result, for this shoulder structure the shoulder clearance gap comprises the sum of the gap between the outside diameter of the washer and cylinder and the gap between the inside diameter of the washer and the tube being retained.
Similarly, the closure can be any of a number of different structures. Where any of these structures includes a washer, normally used adjacent the seal, the inner diameter of the washer is usually smaller than any other component of the closure. Thus, the inner diameter of the washer is the effective diameter of the closure hole. The preselected closure clearance equals the sum of the gap between the inner diameter of the washer and outer diameter of the tube and the gap between the outer diameter of this washer and the diameter of the main cylinder.
For some applications of this invention the body includes a cylindrical guide cavity located within it next to the main cavity to receive the end of the tubing inserted into the fitting. The guide cavity has a diameter sufficiently greater than the diameter of the tubing to form a guide gap of the size which also allows side play to occur to an extent which minimizes fatigue on the tubing within the cavity.
This invention does not reside in any one of the features of the tube fitting disclosed above which is more fully discussed in the Description of the Preferred Embodiment and claimed below. Rather, this invention is distinguished from the prior art by its combination of structural features which make up a unique tube fitting. Important features of this invention are shown and described below to illustrate the best mode contemplated to date of carrying out this invention.
Those skilled in the art will realize that this invention is capable of embodiments which are different from those shown in that the details of the structure of the tube fitting can be changed in various manners without departing from the scope of this invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and are not to restrict the scope of the invention. Additionally, the claims are to be regarded as including such equivalent tube fittings as do not depart from the nature and scope of this invention.